Shafts serving as power transmission shafts for use in, for example, a drive shaft of an automobile are roughly classified into a solid shaft obtained by processing a solid bar, and a hollow shaft obtained by processing a steel tube or the like in terms of structure. In recent years, there is a need for functional enhancement, such as reduction in weight of a suspension system of an automobile and increase in torsional rigidity and NVH characteristic. Accordingly, the hollow shaft is being increasingly used.
FIG. 1 is an illustration of a drive shaft for a front wheel of an automobile using the solid shaft. A drive shaft 30 includes, as main components, a fixed type constant velocity universal joint 1 arranged on a drive wheel side (the left side of the drawing sheet: hereinafter also referred to as an outboard side), a plunging type constant velocity universal joint 31 arranged on a differential side (the right side of the drawing sheet: hereinafter also referred to as an inboard side), and a solid shaft 12 connecting both the constant velocity universal joints 1 and 31 to each other to enable transmission of torque therebetween. A wheel is steered at the drive shaft 30 for a front wheel, and hence as illustrated in FIG. 1, normally, the fixed type constant velocity universal joint 1, which is capable of forming a large operating angle but is not displaced axially, is used on the outboard side (wheel side), whereas the plunging type constant velocity universal joint 31, which forms a relatively small maximum operating angle but is capable of being displaced axially while forming the operating angle, is used on the inboard side (differential side).
The fixed type constant velocity universal joint 1 is a so-called Rzeppa constant velocity universal joint. The constant velocity universal joint 1 includes, as main components, an outer joint member 2, an inner joint member 3, balls 4, and a cage 5. The plunging type constant velocity universal joint 31 is a so-called double-offset constant velocity universal joint. The constant velocity universal joint 31 includes, as main components, an outer joint member 32, an inner joint member 33, balls 34, and a cage 35. Male splines (including serrations, the same holds true for the following description) 19 and 49 formed in both ends of the solid shaft 12 are respectively connected to a female spline 17 formed in the inner joint member 3 of the fixed type constant velocity universal joint 1, and a female spline 47 formed in the inner joint member 33 of the plunging type constant velocity universal joint 31.
FIG. 13 is an illustration of a drive shaft for a front wheel of an automobile using the hollow shaft. Similarly to the above description, the Rzeppa constant velocity universal joint 1 as a fixed type constant velocity universal joint is used on an outboard side of a drive shaft 60. A configuration of the fixed type constant velocity universal joint 1 is the same as that illustrated in FIG. 1, and hence description thereof is omitted. A tripod constant velocity universal joint 61 as a plunging type constant velocity universal joint is used on an inboard side of the drive shaft 60. The tripod constant velocity universal joint 61 includes, as main components, an outer joint member 62, a tripod member 63 serving as an inner joint member, and rollers 64. The rollers 64 are fitted in a freely rotatable manner to three leg shafts 65 formed on the tripod member 63, and the rollers 64 are accommodated in a freely rollable manner in track grooves 66 formed in the outer joint member 62. Male splines 19 and 79 formed in both ends of a hollow shaft 72 are respectively connected to the female spline 17 formed in the inner joint member 3 of the fixed type constant velocity universal joint 1, and a female spline 77 formed in the tripod member 63 of the plunging type constant velocity universal joint 61.
Irrespective of the solid shaft or the hollow shaft, there are a variety of shapes of an end portion (root portion) of the male spline of the shaft on an opposite side to an axial end thereof. Examples of the shape include a shape obtained by simply hollowing out, as illustrated in FIG. 25a, an outer peripheral surface of a power transmission shaft 100 to form a tooth bottom (also referred to as a valley portion) 102 of a male spline 101 formed in the power transmission shaft 100 (hereinafter referred to as a hollowed-out shape), and a shape obtained by smoothly increasing, as illustrated in FIG. 25b, the diameter of the tooth bottom 102 of the male spline 101 to be continuous with the outer peripheral surface of the power transmission shaft 100 (hereinafter referred to as an upward slope shape). In a case of the upward slope shape of the above-mentioned shapes, a diameter increasing surface 105a provides a stress lessening effect, and it is known that the upward slope shape can increase strength of the power transmission shaft.
Further, examples of the upward slope shape include a shape obtained by increasing a circumferential width of the tooth bottom 102 in an axial region of the diameter increasing portion 105 of the male spline on the opposite side to the axial end (hereinafter referred to as a spear shape), and a shape obtained by forming a constant circumferential width of the valley portion in the axial region of the diameter increasing portion (hereinafter referred to as a boat shape). In the spear shape and the boat shape, there is formed a blunt section for blunting an edge of a corner portion between the diameter increasing surface 105a and a tooth flank 104 adjacent to the diameter increasing surface 105a. With this configuration, stress concentration on the corner portion is lessened, thereby increasing static strength and fatigue strength of the power transmission shaft. Such configuration is disclosed in Patent Document 1.